Basic Telescope Features: A Beginner’s Guide

If you are a beginner to the world of telescopes, you may have noticed that there are many terms thrown around when it comes to describing them. Unless you have a basic grasp of what these terms mean, it can be difficult to decide which telescope to purchase, or what type of performance to expect from your telescope. Fortunately, while there are many parts of a telescope, there are only a few that you need to understand in order to purchase a solid beginner’s scope. Following is a description of a few of the most basic terms you will encounter that are used to describe the parts of a telescope.

Types of Telescopes

While all telescopes serve the same basic purpose, they do not all do so in the same way. Instead, there are two basic types of telescopes: Refractors and reflectors. They each process the light that comes through the scope differently, and each offer their own unique advantages and disadvantages. Following is a brief overview of each of these types of telescopes.

Refractor Telescopes

Refractor telescopes are telescopes that use a lens in the front of the optical tube to gather light. The lens bends the light as it passes through (a process called refraction). These telescopes tend to be more durable, stay clean, and give clearer images than reflector telescopes. However, they also tend to be more expensive and to distort color more than reflector telescopes.

Reflector Telescopes

Reflector telescopes use a mirror at the back of the optical tube to gather light. The light bounces off the mirror. This bouncing is called reflection. These telescopes tend to be less expensive and to provide more accurate color than do refractor telescopes. They are especially good choices for larger scopes. However, their pieces tend to lose their alignment over time and to need adjustment more often than do refractor telescopes.

Compound Telescopes

Compound telescopes use both mirrors and lenses to gather the light that comes in the telescope. Cassegrain-Schmidt and Maksutov scopes are examples of compound telescopes. These telescopes tend to be compact and to weigh less than refractor telescopes and reflector telescopes. However, they can also lose focus more easily than other types of telescopes and sometimes do not gather light as efficiently.


Telescopes consist of two main parts: The mount and the optical tube. The mount is the base upon which the optical tube sits. Mounts can be tripods, but are often sturdier bases, due to the weight of the telescope. Mounts come in many different forms and with many different features. While there are a number of different types of mounts, the most common are the altitude-alzimuth (alt-az) and the equatorial. Following is a brief description of each:

Alt-Az Mount

The alt-az mount is the most basic type of mount and is found in Dobsonian telescopes. It allows the optical tube to move up or down or from side to side, much like photo tripods.

Equatorial Mount

The equatorial mount is a more complicated type of mount. It aligns its axis with the Earth’s rotation. As a result, it is easier to track objects in the sky with this type of mount. However, the equatorial mount does not perform as well as the alt-az mount for larger telescopes.

Besides choosing either an alt-az mount or an equatorial mount, you may also find that mounts come with technology that makes them easier to use. For instance, some of them come with software that allows you to direct them to certain areas in the sky. This software can sometimes even locate certain objects in the sky for you. While a bit harder to set up, these mounts can sometimes make it simpler to locate the objects you wish to observe.

Optical Tube

The optical tube is the round part of the telescope that sits on the mount. It contains all of the lenses and mirrors that gather the light and process it into the images of the nighttime sky that you see when you use the telescope. The exact positioning of the internal pieces depends upon the type of telescope you are using. For instance, if you are using a refracting telescope, the objective lens will be in the front, while the objective mirror will be in the back for a reflecting telescope. In addition, the size of the tube often depends upon the size of the lens/mirror. The larger they are, the larger the optical tube tends to be, although there are powerful telescopes that are also compact and portable.

Objective Lens or Mirror

Inside the optical tube are the lenses and mirrors that gather and focus the light that comes into the telescope. The type of lenses and mirrors and their arrangement determine some of the features of the telescope. Some of these features include its light gathering ability, its magnification ability, the clarity of its images, the amount of color distortion it causes, and so forth. The most important element within the optical tube is the objective lens (in a refracting telescope) and the objective mirror (in a reflecting telescope).

The objective lens or mirror (also called the primary lens or mirror), is the first lens or mirror that impacts the light coming into the telescope. In a refracting telescope it bends the light, while in a reflecting telescope it bounces the light. The objective lens or mirror has an opening called an aperture. The size of the aperture determines the light gathering ability of the telescope. In general, the larger the aperture, the more light the telescope can gather. The more light the telescope gathers, the more objects it can see in the nighttime sky. In a reflecting telescope, the light will be focused even more through the use of a secondary mirror.


The focuser is mounted outside the optical tube and is used to further focus the image produced by the telescope. The best focusers are those that allow for tiny adjustments that allow you get the type of clear, sharp image you need.

For beginners, there are two main types of focusers: Rack and pinion focusers and Crayford focusers. Rack and pinion focusers are the most basic of the focusers found on beginner-level telescopes. Crayford focusers, however, are generally thought to provide sharp images with less adjustment and work than rack and pinions. Due to these advantages, Crayford focusers have gradually taken over rack and pinion focusers in popularity in amateur telescopes.


The eyepiece is also called the telescope’s ocular. It fits into the telescope’s focuser to provide greater magnification and field of view than you can get with your telescope alone. One or two eyepieces are sometimes included with a telescope in the purchase. However, there are many different types of eyepieces, and you will probably find that you need to purchase a number of them in order to get the views you want.

Eyepieces work by further focusing the light that comes through the telescope. They provide magnification of images over and above the magnification that the telescope itself provides. Each eyepiece comes with a certain focal length, and the magnification they offer can be calculated by dividing the telescope’s focal length by the focal length of the eyepiece. The maximum magnification you should achieve for your telescope is determined by multiplying the focal ratio of your telescope by 7.

In addition to providing certain levels of magnification, eyepieces also differ from each other by their diameters. They are typically either 1.25 in. in diameter or 2 in. in diameter. They also each come with a certain field of view, which tells you how much of the sky you can see at once while using the eyepiece. There are many different types of eyepieces, from Plossls to Kellners. However, when choosing your eyepiece, the most important aspects to think about are whether it will fit into your focuser, what its magnification is, and what its field of view is.

Finally, when selecting an eyepiece, you should also consider purchasing a Barlow lens to go along with it. This tool, which fits behind the eyepiece in the focuser, can increase your magnification. The result is a better overall viewing experience without requiring you to purchase a new eyepiece.

Telescopes can seem like miraculous objects because of their ability to bring the nighttime sky to life. When you can view images such as the moon, nebulae, or planets, you may wonder how a single machine can help you see such far away sights. However, behind these abilities lies careful craftsmanship and many different parts working together as a unified whole. Understanding some of the primary parts of the telescope can help you get a better grasp of how a telescope works and help you better understand what each telescope you look at can offer you. For instance, you can learn what type of telescope you are looking at, what its basic pieces are, how they function, etc. As a result, you may be able to purchase a better telescope for a more rewarding viewing experience.

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